Research in the Influenza Laboratory is both applied and basic. The focus of the research is support of the mission of the Center for Biologics Evaluation and Research in ensuring the safety, efficacy and availability of influenza virus vaccines. Emphasis is placed on improving the understanding of influenza viral replication and using that understanding to support vaccine development and production. Collaborative studies with other investigators are designed to evaluate virus-host interactions related to safety of influenza vaccines, to prepare for the next pandemic of influenza, and to refine recommendations for use of influenza vaccines. Ongoing applied research projects target development of high growth reassortant influenza A and B viruses both for current vaccines and in order to prepare for potential pandemics in the event new influenza A subtypes are introduced in human populations. Basic research is directed at understanding the contribution of specific viral proteins in control of replication and assembly of influenza viruses, and investigating how changes in those proteins can be used to alter the growth characteristics in mammalian and avian systems. A variety of techniques is used in the research including standard egg and tissue culture viral procedures as well as amplification, sequencing and cloning of specific viral genes, transient and stable expression of viral proteins and RNA in cells, and other molecular and immunologic methods. Although most recent strains of influenza viruses were well inhibited in antibody studies using sera from people immunized with current influenza vaccines, information collected during late February through March suggested continued antigenic drift in influenza A H3N2 viruses. Since no influenza A H3N2 strain that could be used to produce a superior vaccine was identified by the time vaccine production needed to begin in order to avoid delay of vaccine availability in the fall, the USPHS (in concert with WHO) recommended that influenza vaccine composition remain unchanged for 2003-2004. Our laboratory produced reference reagents for use in standardizing inactivated influenza vaccines to be used during 2003-2004, and these reagents facilitated qualification and release of approximately 85 million doses of influenza vaccine for the United States. Avian influenza A H5 subtype viruses were shown to infect 2 people in China during February 2003. Since these strains demonstrated antigenic drift from earlier H5 viruses, we have begun to develop additional reagents to be used for testing experimental vaccines produced against the drifted H5 influenza A viruses. Most influenza B viruses replicate poorly in eggs, but by serial passage they can sometimes be adapted to produce higher yields in eggs. Our laboratory has investigated changes in influenza B viruses after adaptation to eggs by serial passage, and has found that mutations altering single amino acids in either an internal viral protein or viral hemagglutinin can significantly increase growth in eggs. In addition, changes affecting the hemagglutinin may result in antigenic alterations making the virus unsuitable as a vaccine candidate. The results suggest that influenza B viruses passaged serially to increase growth potential should be routinely tested for antigenic characteristics before they are used for vaccine production. Further investigations are planned.